Like in round 4 (since the related code hasn't changed) the many concurrent spawned goroutines probably get in each others way, causing a high latency and low throughput.
There was a reversion of the test without goroutines, which I think performed better. But I was told the goroutines version is more realistic... (I don't share this opinion).
To be fair, this version had also a manual connection pool to address a previous bug.
Also Go's database connectivity is not very mature yet. There is still a lot of work to do. I'm pretty sure it can and will be done.